Up to now, in the field of semiconductors, a barrier metal film layer has been formed. Such a barrier metal film layer has conventionally been formed, in the form of a thin film between a wiring films and insulating films. Such a barrier metal film layer has conventionally been formed in the form of a thin film according to, for instance, the sputtering technique. It has therefore been required for the barrier metal film layer to be formed with a thinner and more uniform thickness. For this reason, it has correspondingly been required for the sputtering target used for forming a barrier metal film layer to make the grain size of the constituent metal finer and more uniform for the purpose of ensuring the uniformity of the resulting film within the plane thereof (so-called plane uniformity) and for the control of the formation of any particulate material during the sputtering operations. In case a film for making Cu wiring is used, it is known to use Ta for forming a barrier metal film layer. Regarding the Ta sputtering target used for forming a Ta barrier metal film layer, it would likewise be effective to make the average grain size of Ta finer and to control the local scattering of the grain size, in order to control the formation of any particulate material during the sputtering operations and to likewise control the occurrence of any scattering in the sheet resistance of the resulting thin film.
To this end, there have variously been investigated the development of methods for preparing a Ta sputtering target, which can make the grain size of the Ta sputtering target finer and can make the average grain size smaller, thereby inhibiting the occurrence of any scattering in the grain size thereof. In order to make the crystalline microstructure of a metal material uniform and finer, it is general practice to perform the following, namely, the sputtering target be produced by setting the processed amount (reduction ratio) at a level as high as possible upon the plastic forming operations. Subsequently, heat-treating conditions are selected to prevent the grain size from getting too large or coarse. As the method for producing the Ta sputtering target, there have been following reports when largely classified.
(1) A method in which the processed amount is increased through the use of the combination of stamping and upsetting forging operations to thereby prepare a forged slab (billet). Thereafter, the slab (billet) is subjected to a vacuum heat treatment and to a cold rolling operation. The rolled plate is subjected to vacuum heat treatment.
(2) A method in which a slab or billet is prepared by a combination of two or more knead-forging operations and vacuum heat treatments. The slab or billet is then subjected to cross-rolling, and then the rolled plate is subjected to vacuum heat treatment (see, for instance, JP-A-1999-80942, JP-A-2004-107758, Int'l Appln. Published under PCT (Tokuhyo) No. JP-A-2004-526863 (WO02/070765 A1), and Int'l Appln. Published under PCT (Tokuhyo) No. 2002-530534 (WO00/31310)).
Ta sputtering targets were produced by way of experiments according to these methods, but these methods are not always sufficient for the preparation of a Ta sputtering target having sufficiently fine crystalline microstructure with small scattering of the average grain size. The inventors of this invention have cleared up the causes of the foregoing drawbacks associated with the above-described methods. They have found that the foregoing drawbacks are caused for the following reasons, namely:
(1) the forging conditions thereof were inadequate and thus the crystalline microstructure of the billet (slab) was not uniform;
(2) the draft percentage at the time of rolling was inadequate (i.e., the draft percentage was small); and
(3) the heat-treating conditions of the plate to be rolled were inadequate.
The reason why the crystalline microstructure of the billet was not uniform can be attributed to the following, namely:
(1) the shape of the ingot (i.e., too large or too small an aspect ratio);
(2) apparatus used for the production (i.e., too small a load of the pressing machine); and
(3) method of forging (i.e., too small the number of axes or too small the number of repeated operations).
For instance, when the length L of the ingot or when the length L at the time of deformation during forging is below 0.5 to 0.6 times the diameter φ thereof, the force during the stamp-forging never easily reaches the central portion of the ingot. Therefore, the resulting forged product is liable to have a concave area at the center thereof or in the vicinity of the center (hereunder referred to as “central portion”) with respect to the side thereof near the outer periphery or outer regions (hereunder referred to as “outer peripheral portion”). As a result, the resulting product has a crystalline microstructure insufficient in the uniformity between the outer peripheral portion and the central portion. If this non-uniformity is more conspicuous, the concave area in the central portion would remain depending on the thickness of the billet even if the subsequent upset-forging operation is carried out and this would become a cause of a considerable reduction in the production yield of such billets. In addition, when the length L of an ingot or the length L observed upon the deformation through the forging is more than 1.5 to 2.0 times the diameter φ thereof in the stamp-forging operation, the ingot may easily undergo buckling and deflection during the upset-forging operation. In other words, in case the aspect ratio (diameter/length) is inadequate with the ingot or at the time of deformation by forging, a stable production by cold forging cannot be made. As a result, there arises a problem in that the uniformity in the crystalline microstructure cannot be attained, i.e., in that the yield is lowered.
The usual sputtering target can be obtained by the rolling of a billet after the forging of a starting ingot and the subsequent heat-treating processes. Even if a billet which is high in uniformity in the crystalline structure is prepared, the subsequent lower draft percentage at the time of rolling will result in a failure to obtain, in the subsequent heat treating, a uniform and crystalline microstructure. On the other hand, even if a billet which is high in uniformity in crystalline microstructure is prepared, too high a draft percentage will give rise to a problem in that the growth into too coarse a grain size of the crystalline microstructure is likely to be accelerated in the subsequent heat treating.